1. Field of the Art
This invention relates to a semiconductor light-emitting device.
2. Prior Art
In a prior art high-speed responding light-emitting diode of double-hetero structure type of In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y /InP, it was a conventional practice to dope p-In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer with Zn which is a p-type impurity in the high density of 3-10.times.10.sup.18 cm.sup.-3 and to make this active layer grow epitaxially on n-type InP clad layer to thereby increase the response speed (see, for examples, Electronics Letters, 1983, Vol. 19 No. 23, pp 963-965 and The Preprints 6P-H-8 for 30th Lectures of Federation of Applied Physics Societies, Spring 1983). In FIG. 1, reference numeral 1 denotes n-InP substrate doped with Te in the density of 1.times.10.sup.18 cm.sup.-3, 5 denotes p-InP clad layer doped with Zn in the density of 3.times.10.sup.18 cm.sup.-3, and 6 denotes p-In.sub.1-W Ga.sub.W As.sub.1-Z P.sub.Z contact layer doped with Zn in the density of 3.times.10.sup.18 cm.sup.-3.
FIG. 2 shows carrier density profile of the layers of the light-emitting diode having the structure shown in FIG. 1. In FIG. 2, numerals 1, 2, 4, 5 and 6 along the horizontal axis correspond to the reference numerals 1, 2, 4, 5 and 6 denoting the layers of the structure of FIG. 1. Since the p-In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 4 is doped with Zn, a p-type impurity, in the high density, Zn is diffused to the n-InP clad layer 2 during the epitaxial growth as shown by the broken line in FIG. 2, to make a portion of the n-InP clad layer 2 p-InP. For this reason, there is caused a remote junction in which the p-n junction is shifted from the interface between the p-In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 4 and the n-InP clad layer 2 into the n-InP clad layer 2.
FIG. 3 shows energy band profile of the layers of FIG. 2. In FIG. 3, reference numerals 1, 2, 4, 5 and 6 along the horizontal axis correspond to the reference numerals 1, 2, 4, 5 and 6, respectively, denoting the layers in FIGS. 1 and 2. When the junction is shifted into the n-InP clad layer 2 as shown in FIG. 3, a portion of the carriers flow out from the p-In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 4 which is the light-emitting region into the n-InP clad layer, to thereby reduce the light-emitting output.
Further, since the remote junction makes the carrier enclosure incomplete, reduction in the response speed is inevitable even when the p-type impurity Zn is doped in a high density.
To avoid the remote junction, it is conventional to provide sequentially the n-InP layer 2 on the n-InP substrate 1, the undoped In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 3 on the n-InP layer 2, a p-InP clad layer 5' doped with Zn in a high density on the active layer 3, and the p-In.sub.1-W Ga.sub.W As.sub.1-Z P.sub.Z contact layer 6 on the p-InP clad layer 5'. FIG. 4 shows carrier density profile of the light-emitting diode having the sequentially provided layers as described above. As shown by a broken line in FIG. 4, a p-n junction is formed in the active layer 3 by diffusing Zn from the p-InP clad layer 5' into the In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 3 during growth of crystal. In the light-emitting device produced by the method described above, however, a half value width of the light-emitting spectrum is enlarged as shown in the curve (b) of FIG. 5. This is becuase, in the above-described method, Zn is diffused substantially over the entire region of the In.sub.1- X Ga.sub.X As.sub.1-Y P.sub.Y active layer 3 so as to distort the crystal of the In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer, or, even in the case where Zn is not diffused over the entire region, the p-n junction is formed in about the middle of the In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 3.
The light-emitting device produced in the method described above is, in spite of its high response speed, not suitable for high speed data transmission through optical fibers because of the large half value width of its light-emitting spectrum. That is, the data transmittable distance through optical fibers depends upon the half value width of the light-emitting spectrum of the light-emitting device as shown in FIG. 6. In the case where the half value width is 160 nm as shown by the curve (b) in FIG. 5, data transmission over a distance of 0.8 km or larger is practically impossible as shown by a broken conventional light-emitting device produced by Zn diffusion as described above has a further disadvantage that uneven Zn diffusion in the In.sub.1-X Ga.sub.X As.sub.1-Y P.sub.Y active layer 3 in the production process causes variation in characteristics of the device.